Our invention relates to a discriminator circuit for frequency modulation (FM) receivers, and particularly to such a discriminator for use with a relative low intermediate frequency.
In a superheterodyne type of radio receiver, rejection of undesirable image radio frequencies requires the use of radio frequency selectivity. For example, if a carrier frequency of 150.030 Megahertz is to be received by a receiver whose first intermediate frequency (IF) is 10.0 Megahertz, the receiver local oscillator might be set at 140.030 Megahertz. However, a received carrier frequency of 130.030 Megahertz (called the image frequency) would also produce the first IF of 10.0 Megahertz. Hence, a selective circuit that rejects the image frequency is very desirable, and in some cases essential.
Accordingly, a primary object of our invention is to provide a new and improved circuit for a superheterodyne radio receiver that rejects many image frequencies.
Another object of our invention is to provide a new and improved circuit for a superheterodyne radio receiver that rejects many image frequencies without the need, and hence expense, of a selective circuit to reject those image frequencies.
One prior art superheterodyne receiver for reducing the image frequency problem uses a single converter and a low intermediate frequency (IF), such as 7 Kilohertz. In the above example, where the received carrier is 150.030 Megahertz, the receiver local oscillator might be set at 150.030 Megahertz minus 7 Kilohertz, or 150.023 Megahertz. If the channel spacing at those carrier frequencies is 30 Kilohertz, there should not be any signals at the image frequency of 150.023 Megahertz minus 7 Kilohertz or 150.016 Megahertz, since the next lower carrier would be 150.000 Megahertz. Hence, the image frequency problem is substantially reduced or eliminated. However, the low IF of 7 Kilohertz makes it difficult to detect intelligence present on the IF, particularly if the intelligence has any appreciable bandwidth. Voice transmissions require a 3 Kilohertz bandwidth for reliable communication under normal conditions. This requirement and an IF of 7 Kilohertz results in a low IF to intelligence bandwidth ratio of 2.3 to 1. Such a low ratio makes detection difficult since the 7 Kilohertz IF must be rejected, and the 3 Kilohertz intelligence must be passed in order to be detected.
Accordingly, another object of our invention is to provide a new and improved FM discriminator for use with relatively low intermediate frequency to intelligence bandwidth ratios.
Another object of our invention is to provide an improved FM discriminator that does not require complex circuits or filters to detect intelligence from an IF with a relatively low IF to intelligence bandwidth ratio.
Another object of our invention is to provide a new and improved FM discriminator that can operate with relatively low voltage and current.